396 THE ORIGIN OF SPECIES 



eral classes are now known to have the power of ordinary repro- 

 duction at an unusually early age ; and we have only to accelerate 

 parthenogenetic reproduction by gradual steps to an earlier and 

 earlier age — Chironomus showing us an almost exactly inter- 

 mediate stage, viz., that of the pupa — and we can perhaps account 

 for the marvellous case of the Cecidomyia. 



It has already been stated that various parts in the Same indi- 

 vidual, which are exactly alike during an early embryonic period, 

 become widely different and serve for widely different purposes in 

 the adult state. So again it has been shown that generally the em- 

 bryos of the most distinct species belonging to the same class are 

 closely similar, but become, when fully developed, widely dis- 

 similar. A better proof of this latter fact cannot be given than the 

 statement by Von Baer that "the embryos of mammalia, of birds, 

 lizards and snakes, probably also of chelonia, are in the earliest 

 states exceedingly like one another, both as a whole and in the 

 mode of development of their parts; so much so, in fact, that we 

 can often distinguish the embryos only by their size. In my pos- 

 session are two little embryos in spirit, whose names I have omitted 

 to attach, and at present I am quite unable to say to what class 

 they belong. They may be lizards or small birds, or very young 

 mammalia, so complete is the similarity in the mode of formation 

 of the head and trunk in these animals. The extremities, however, 

 are still absent in these embryos. But even if they had existed in 

 the earhest stage of their development we should learn nothing, 

 for the feet of lizards and mammals, the wings and feet of birds, 

 no less than the hands and feet of man, all arise from the same 

 fundamental form." The larvae of most crustaceans, at correspond- 

 ing stages of development, closely resemble each other, however 

 different the adults may become; and so it is with very many other 

 animals. A trace of the law of embryonic resemblance occasion- 

 ally lasts till a rather late age: thus birds of the same genus, and 

 of allied genera, often resemble each other in their immature 

 plumage; as we see in the spotted feathers in the young of the 

 thrush group. In the cat tribe, most of the species when adult are 

 striped or spotted in lines; and stripes or spots can be plainly 

 distinguished in the whelp of the lion and the puma. We occa- 

 sionally, though rarely, see something of the same kind in plants ; 

 thus the first leaves of the ulex or furze, and the first leaves of the 

 phyllodineous acacias, are pinnate or divided, like the ordinary 

 leaves of the leguminosae. 



The points of structure, in which the embryos of widely differ- 

 ent animals within the same class resemble each other, often have 



